In a known conventional technology, temperature is measured by detecting and using the frequency shift of Brillouin scattered light outputted from an optical fiber to which light is inputted (Non-patent reference 1). Brillouin scattering is a light scattering phenomenon caused by interaction between light in an optical fiber and sound waves in the optical fiber. The technology described in Non-patent reference 1 below utilizes changes in the spectrum of Brillouin scattered light with respect to the temperature of the optical fiber.
Reference 1 below, in particular, describes that the frequency shift of Brillouin scattered light changes linearly with respect to temperature in a temperature region around 230 to 370K. Reference 2 below, also, describes that frequency shift takes an extreme value around 60K to 90K.    Non-patent reference 1: Marc Nikles et al. “Brillouin gain spectrum characterization in Single-Mode optical fibers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 15, NO. 10, October 1997    Non-patent reference 2: L. Thevenaz et al., “Brillouin gain spectrum characterization in optical fibers from 1 to 1000K”, Technical Digest, 16th International Conference on Optical Fiber Sensors, Oct. 13-17, 2003, Tu 2-2, p. 38-41